U.S. patent number 5,688,318 [Application Number 08/468,971] was granted by the patent office on 1997-11-18 for photonic band gap materials and method of preparation thereof.
Invention is credited to Joseph B. Milstein, Ronald G. Roy.
United States Patent |
5,688,318 |
Milstein , et al. |
November 18, 1997 |
Photonic band gap materials and method of preparation thereof
Abstract
The invention concerns materials which exhibit photonic band
gaps in the near infrared and visible regions of the optical
spectrum and methods of preparation of such materials.
Inventors: |
Milstein; Joseph B. (Brighton,
MA), Roy; Ronald G. (Tewksbury, MA) |
Family
ID: |
25532822 |
Appl.
No.: |
08/468,971 |
Filed: |
June 6, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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266146 |
Jun 27, 1994 |
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986848 |
Dec 4, 1992 |
5385114 |
Jan 31, 1995 |
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Current U.S.
Class: |
117/1 |
Current CPC
Class: |
H01L
31/18 (20130101); B82Y 20/00 (20130101); G02F
1/21 (20130101); G02F 1/0131 (20130101); H01L
31/1804 (20130101); G02B 6/1225 (20130101); Y02P
70/50 (20151101); G02F 2202/32 (20130101); Y02E
10/547 (20130101); Y02P 70/521 (20151101) |
Current International
Class: |
G02B
6/122 (20060101); H01L 31/18 (20060101); C30B
033/06 () |
Field of
Search: |
;117/1,54,74,75,81,87,95
;437/127,126,129 ;372/39,43 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Garrett; Felisa
Parent Case Text
This is a division of application Ser. No. 08/266,146 filed 27 Jun.
1994, which was itself a division of application Ser. No.
07/986,848, filed 4 Dec. 1992, now U.S. Pat. 5,385,114, issued 31
Jan. 1995.
Claims
What is claimed is:
1. Modification of a photonic band gap behavior of a photonic band
gap material by modification of a physical periodicity or a
dielectric constant or both of the said photonic band gap material
by application of one or more of the excitations chosen from the
group comprising:
a. a thermal excitation,
b. a physical compression or extension,
c. an electric field excitation, and
d. a magnetic field excitation, or by the addition of the same or
other photonic band gap material so as to modify the physical
periodicity of the said photonic band gap material.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to photonic band gap (PBG) materials
and method and means of preparation of the same. A definition of
the concept of a photonic band gap material is in order, and may be
stated as follows. In direct conceptual analogy to the appearance
of an electronic band gap in a semiconductor material, which
excludes the possibility that electrical carriers can have
stationary energy states within the band gap, one can theoretically
postulate the appearance of a photonic band gap in a dielectric
medium in which the possibility of stationary photonic energy
states (i.e., electromagnetic radiation having some discrete
wavelength or range of wavelengths) must be rigorously excluded in
that band gap. In semiconductors, the electronic band gap results
as a consequence of having a periodic atomic structure upon which
the quantum mechanical behavior of the electrons in the material
must attain eigenstates. By analogy, the photonic band gap results
if one has a periodic structure of a dielectric material where the
periodicity is of a distance suitable to interact periodically with
electromagnetic waves of some characteristic wavelength that may
appear in or be impressed upon the material, so as to attain
quantum mechanical eigenstates.
In particular, one class of uses of these materials that can be
envisioned, but which so far has not been practically demonstrated,
is the optical analog to semiconductor behavior, in which a
photonic band gap material, or a plurality of such materials acting
in concert, can be made to interact with and control light wave
propagation in a manner analogous to the way that semiconductor
materials can be made to interact with and control the flow of
electrically charged particles, i.e., electricity, in both analog
and digital applications.
2. Description of the Prior Art
A review of possible applications of these materials is presented
by Henry O. Everitt in an article entitled, "Applications of
Photonic Band Gap Structures", in Optics add Photonics News, volume
3, number 11, pages 20-23, which was published in November,
1992.
Prior descriptions of photonic band gap materials and methods of
preparation of these materials include those of Eli Yablonovitch
and co-workers, which have appeared in articles in the scientific
journal Applied Physics Letters. These authors have demonstrated
that the concept of a photonic band gap material is valid, but have
only shown this behavior in the microwave region of the
electromagnetic spectrum. They discuss conceptual methods of
manufacturing such materials, which rely on the removal of
predetermined volumes of material from a solid mass of a uniform
material.
These prior inventions have never been applied in commercial
practice, but have been described in the scientific literature
only, as far as we have been able to determine.
We will present an example which demonstrates the superiority of
the present invention over the prior art in photonic band gap
materials. The major applications of photonic band gap materials
are likely to be in the areas of the use and control of
electromagnetic radiation in the wavelength range extending from
the millimeter or microwave region to the ultraviolet region. The
prior descriptions of methods that may be employed to fabricate
such materials typically involve the mechanical drilling or
machining of holes or cavities of macroscopic dimensions (of the
order of millimeters or tenths of millimeters) in solid blocks of a
dielectric material, or the concept of using physically directed
and orientationally controlled chemical removal such as reactive
ion etching to fabricate holes or cavities having dimensions of the
order of microns in solid blocks of a dielectric material. These
procedures suffer from the disadvantages that they are time
consuming, expensive to perform, and require sophisticated and
expensive machinery for their practice. Everitt, in the November
1992 publication cited above, states on page 23:
"However, a number of issues must be addressed before the crystals
can live up to expectations. Fabrication difficulty increases with
increasing band gap frequency. Materials with high real dielectric
constants and low loss tangents must be identified for all
frequency regions. Theories describing pure and doped PBG crystals
must be refined and optimal structures identified. Finally the
intolerance of the crystals to imperfections and the lack of
post-fabrication gap/defect tunability are practical concerns an
experimenter must face.
Nevertheless, given that PBG crystals were first proposed five
years ago and demonstrated only last year, researchers are
optimistic that these obstacles can be mitigated. For the
interested reader, a more in-depth survey of current activity
involving PBG crystals and their potential applications may be
gleaned from an upcoming issue of JOSA B dedicated to the
"Development and Application of Materials Exhibiting Photonic Band
Gaps," (February 1993)."
JOSA B refers to the Journal of the Optical Society of America,
part B.
Our invention Provides a solution to these problems, and is, as far
as we can determine, the first to demonstrate photonic band gap
behavior in the near infrared and visible portions of the
electromagnetic spectrum.
In addition, we have discovered other new details of construction,
which, when taken together, permit our invention to achieve results
which the previously disclosed technologies are not capable of
achieving. These discoveries will be stated explicitly in the
discussion of the invention.
Yablonovitch et. al. teaches a method and means for the
construction of a photonic band gap material in Applied Physics
Letters volume 67, number 17, pages 2295 to 2298, published on Oct.
21, 1991, in which it is stated, on page 2296:
"A slab of material is covered by a mask containing a triangular
array of holes. Three drilling operations are conducted through
each hole, 35.26.degree. off normal incidence and spread out
120.degree. on the azimuth. The resulting crisscross of holes below
the surface of the slab provides a fully three-dimensional periodic
fcc structure, with WS unit cells . . . The drilling can be done
with a real drill bit for microwave work, or by reactive ion
etching to create an fcc structure at optical wavelengths. We have
fabricated such crystals in the microwave region by direct drilling
into a commercial, low-loss, dielectric material, Emerson and
Cumming Stycast-12. Its microwave refractive index, n.about.3.6, is
meant to correspond to the that of the common semiconductors, Si,
GaAs, etc. By simply scaling down the dimensions, this structure
can be employed equally well at optical wavelengths."
In an article entitled "Photonic Band Gaps", published in the
August, 1992 issue of Physics World, pages 37 to 42, Philip St. J.
Russell states, (page 37),
"Despite the difficulties of designing and constructing the right
kind of structure, and of detecting what happens, Yablonovitch's
team have observed a PBG at microwave frequencies in a specially
drilled dielectric material (microwave refractive index of 3.6)
with a face-centered-cubic lattice. This "dielectric crystal" was
produced by drilling evenly spaced (8 mm pitch) sets of holes at
three carefully chosen angles. To obtain a band gap at optical
frequencies requires a very much smaller lattice spacing
(.about.400 nm for 1.5 .mu.m light in GaAs) and is much more
challenging to produce. Although techniques involving reactive ion
beam etching are being actively developed, no success has yet been
reported."
Since Yablonovitch's device is fabricated by the removal of a
selected portion of the slab of the starting material, it is clear
that the present invention differs significantly and qualitatively
from the device of Yablonovitch with regard to its underlying
principles of fabrication. Each of the prescriptions in
Yablonovitch's papers is based specifically on this geometry and
method of construction, in which the structure is fabricated from a
solid slab of the starting material from which material is
removed.
A second type of structure having holes or pores has also been
fabricated by an etching process applied to single crystal silicon.
Recently, L. T. Canham at the Royal Signals and Radar Establishment
(RSRE) in the United Kingdom reported the ability of anodized
crystalline silicon ("porous" silicon) wafers to emit light in the
visible under illumination, with no electrical contacts attached
(Applied Physics Letters 57, 1046 (1990)). This result, which has
been confirmed by other groups, is not well understood but is
reproducible. It appears to depend on the fabrication of "quantum
wires", having diameters measured in nanometers and lengths of some
microns. According to RSRE, electrochemical and chemical
dissolution are used to etch a thin layer of free-standing wires in
the surface of bulk silicon wafers. While this etching process is
not yet understood, it produces these wires rather than merely
removing some thickness of silicon in a uniform manner. The
dimensions of the wires may be related to the wavelength of the
light which is emitted. Under the action of a blue (488 nm) laser,
in excess of one percent of the incident light can be emitted in
the green, while under green laser light emission in the red is
observed. Both of these findings suggest that relatively high
absorption efficiency should be possible. Some of these results
have appeared in the literature.
In contradistinction to Yablonovitch and to Canham, we have
discovered that it is possible to fabricate a photonic band gap
material by impregnating the pores or voids contained within the
volume of a specially prepared reticulated mesh, which may be made
of a material with a high melting temperature such as a metal, with
liquid material which melts at a temperature lower than the melting
temperature of the reticulated mesh and which solidifies upon
cooling. The reticulated mesh is then dissolved by simple chemical
action in a liquid bath, leaving behind a solid reticulated
structure composed of the solidified liquid material. In
particular, the liquid material may be caused to solidify into an
ordered solid such as a single crystal by the imposition of either
or both a thermal gradient or a seed single crystal of the same or
a closely related material. Rather than being a subtractive
process, such as those of Yablonovitch or Canham, in which material
is removed from a single mass, our process is an additive process
in which a structure is produced by solidifying or adding material
to form a structure. A template can be employed which may or may
not be removed, as may be required, after the additive process is
carried out.
Many embodiments of the present invention can be envisioned. In one
instance, the material can be made as a reticulated single crystal
of a material having a high dielectric constant, such as sapphire.
In another instance, the material could be made as a
polycrystalline reticulated solid of a material having a high
dielectric constant. Other embodiments could be made from a
reticulated single crystal of doped material having high dielectric
constant, such as ruby (sapphire doped with chromium in 3+
oxidation state) or titanium-doped sapphire, both of which are in
themselves laser materials. Yet other embodiments could be made
from reticulated doped glass, such as neodymium doped glass, which
is also a laser material. Yet further embodiments of the invention
can be recognized in which a second material, having a second,
different dielectric constant and different optical behavior, is
introduced into the voids of the reticulated solid material. Still
further embodiments can be suggested which may consist of
reticulated bodies of material having high dielectric constant with
pore sizes of dimensions significantly larger or smaller than
approximately 10 microns, for example in the range of approximately
200 microns to approximately 1 micron or perhaps less, if the
reticulated metal structures can be made, for example by the
freezing of electrohydrodynamically generated droplets or mist. Yet
further embodiments can be pointed out, as regards the means of
impregnation of the fluid into the reticulated template solid,
which can include capillary action, as when a liquid such as water
is blotted up by tissue paper, or which can include a pressure
differential, as when a gas is allowed to fill a volume, either by
raising the pressure at the entry or diminishing the pressure at
the exit, or a combination of both.
In addition, photonic band gap materials and devices constructed
according to the prescriptions given in the present invention may
be used in optical equipment and machines of more advanced design
than those manufactured previously.
SUMMARY OF THE INVENTION
A specific example which we have investigated is the fabrication of
a sapphire photonic band gap material in single crystal form,
having continuous pores of an average size of approximately 10
micrometers in diameter as measured by scanning electron
microscopy. This material was generated by infusion of liquid
aluminum oxide by capillary action into a reticulated cylindrical
body of tungsten, permitting the aluminum oxide to solidify, and
etching away the tungsten body in a solution consisting of a
mixture of aqueous hydrofluoric and nitric acids. A reticulated
aluminum oxide (sapphire) body resulted which behaved as a photonic
band gap material in the near infrared and visible portion of the
electromagnetic spectrum. This reticulated body can be thought of
as the three dimensional "negative" of the tungsten reticulated
body, in that it has continuous pores where the tungsten was solid,
and is solid where the tungsten had continuous pores.
The photonic band gap was observed as follows. The reticulated
sapphire body was subjected to heating by the flame of a gas
(hydrogen-oxygen) torch in a daylight environment. At room
temperature, the reticulated sapphire body was white in color,
similar to white chalk used for writing on a chalkboard. As the
temperature of the body was increased by the flame, it suddenly
became black in color. It remained black in color for some time
until the temperature was raised by an amount, at which time it
began to glow with a reddish to orange color. Upon cooling, the
body again suddenly changed in color from reddish or orange to
black, and finally after some time again changed suddenly to white
as it came down to room temperature. One notes that black is the
response observed in the absence of light. This optical behavior is
completely consistent with the expected behavior of a photonic band
gap material having its optical band gap in the near infrared to
visible portion of the electromagnetic spectrum.
The optical behavior can be explained as follows. It is well known
from quantum physics that any solid object radiates electromagnetic
radiation according to Planck's Law of Radiation, which states that
the intensity of radiation emitted as a function of wavelength and
the body's temperature is proportional to the quantity, Q1,
where ##EQU1## k=Boltzmann's constant=1.38044.times.10.sup.-16
erg/degree .lambda.=wavelength
T=temperature in Kelvin and the wavelength at which the maximum
radiation intensity occurs as a function of temperature is given by
.lambda.A.sub.max
where
.lambda..sub.max =hc/(4.965kT)=0.2897/T centimeters with h, c, k,
and T as above.
Hence, a body would have its maximum emission at a wavelength of
0.2897/298 centimeters, or 9.72 micrometers (microns), at a
temperature of 298K or room temperature. It is well known from
experience that a body at about 700.degree. Centigrade normally is
beginning to be hot enough to be visible with a dull red color and
that at a temperature of approximately 900.degree. to 1000.degree.
Centigrade, a body looks reddish to orange. The range of response
of the human eye to visible light is about 0.7 microns in the red
to 0.4 microns in the violet. At 700.degree. Centigrade, or 973K,
the maximum intensity would occur at 0.2897/973 centimeters, or 2.9
microns. At 1000.degree. Centigrade, the maximum intensity has
shifted to 0.2897/1273 centimeters, or 2.28 microns. Thus our
material exhibits the exclusion of light in a range which can be
estimated as extending from a few microns to the red portion of the
spectrum, which is exactly what a photonic band gap material in the
near infrared to the visible should do.
In the present invention, a material which operates in the near
infrared and the visible as a photonic band gap material has been
discovered. None of the difficulties presented in the literature
regarding the fabrication of the material are impediments to its
fabrication by the method and means we have discovered. A
significant improvement in the ease and speed of fabrication has
resulted.
The general purpose of this invention is to provide a photonic band
gap material and the method and means for its fabrication, which
has all of the advantages of similarly employed technology and has
none of the above described disadvantages. To attain this, the
present invention provides a unique class of photonic band gap
material, and method and means for its fabrication.
An object of the instant invention is to provide method and means
of construction of a photonic band gap material which has all of
the advantages of similarly employed technology and has none of the
above described disadvantages. To attain this, the present
invention provides a unique method and means of constructing a
photonic band gap material.
Another object of the instant invention is to provide method and
means of construction of a photonic band gap material which permits
the fabrication of a photonic band gap material which is itself
composed of reticulated single crystal material.
Yet another object of the instant invention is to provide method
and means of construction of a photonic band gap material which
permits the fabrication of a photonic band gap material which is
composed of a reticulated doped material.
Still another object of the instant invention is to provide method
and means of construction of a photonic band gap material which
permits the fabrication of a photonic band gap material which has
pores of predetermined sizes over a wide range of sizes, from
approximately 200 microns to approximately 1 micron or less.
It is another object of the instant invention to provide method and
means of construction of a photonic band gap material which has a
multiplicity of pore sizes in selected and distinct regions of its
structure.
It is still another object of the instant invention to provide
method and means of construction of a photonic band gap material
which can be physically small or large in size, ranging from
external dimensions of less than a millimeter to greater than
centimeters, and perhaps as large as multiple tens of
centimeters.
It is still a further object of the instant invention to provide
method and means of construction of a photonic band gap material
which can be infiltrated with a second, dissimilar material having
a different dielectric constant.
It is yet another object of the instant invention to provide method
and means of construction of a photonic band gap material which
operates in the infrared region of the electromagnetic
spectrum.
It is still yet another object of the instant invention to provide
method and means of construction of a photonic band gap material
which operates in the visible region of the electromagnetic
spectrum.
It is still yet another object of the instant invention to provide
method and means of construction of a photonic band gap material
which operates in the ultraviolet region of the electromagnetic
spectrum.
These and other objects and features of the present invention will
be apparent to those skilled in the art from the following
descriptions of specific embodiments of the invention taken in
conjunction with the figures.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and many of the attendant advantages of this
invention will be readily appreciated as the same becomes better
understood by reference to the following detailed description when
considered in connection with the accompanying drawing, in which
like reference numerals designate like parts throughout the figures
thereof and wherein:
FIG. 1 is a perspective view of the porous single crystal photonic
band gap material embodying the present invention.
FIG. 2 is a cutaway sectional view taken through the growth station
used to prepare the photonic band gap material embodying the method
and means of the present invention, wherein infiltration of
photonic band gap material from a liquid source into a template
solid takes place.
FIG. 3a is a cutaway sectional view taken through the growth
station used to prepare the photonic band gap material embodying an
alternative method and means of the present invention, wherein
infiltration of photonic band gap material from a gaseous source
into a template solid takes place with complete filling of the
pores in the template solid.
FIG. 3b is a cutaway sectional view taken through the growth
station used to prepare the photonic band gap material embodying
the alternative method and means of the present invention, wherein
infiltration of photonic band gap material from a gaseous source
into a template solid takes place with partial filling of the pores
in the template solid.
FIG. 4 is a sectional view taken through a rectangular slab of
photonic band gap material composed of three planar regions, the
two outermost regions having one composition of matter, and the
third central region having a different composition of matter,
which is fabricated as a single continuous object.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, the photonic band gap material is comprised of
a reticulated solid 10 which forms a cylinder. In FIG. 1, the solid
portion of the photonic band gap material is shown in black., and
the voids in the photonic band gap material, which extend
substantially and continuously throughout the body of the photonic
band gap material, are shown in white. The photonic band gap
material may be a high dielectric constant substance, such as
aluminum oxide or sapphire, other oxides such as ruby
(chromium-doped aluminum oxide), yttrium aluminum garnet (YAG) or
other similar synthetic garnets, semiconductors such as elemental
silicon, germanium and the like, semiconductors formed as compounds
from columns III and V of the periodic table such as gallium
arsenide, indium phosphide, and similar compound semiconductors or
ternary or higher order alloys of these semiconductors, such as
gallium aluminum arsenide, semiconductors formed as compounds from
columns II and VI of the periodic table such as zinc selenide, zinc
sulphide, cadmium telluride, mercury telluride and similar compound
semiconductors or ternary or higher order alloys of these
semiconductors, such as mercury cadmium telluride, or materials
having high dielectric constant which can be infiltrated into a
porous body and then solidified, such as epoxies. The solid is
shown as a cylinder simply because this geometry is a convenient
one for use in a conventional optical system involving the
propagation of collimated light beams. As will become apparent from
the description of the fabrication of the solid, other geometrical
forms can readily be fabricated. The pores in the reticulated solid
are all of approximately the same size, and are defined by the
process used to generate the reticulated solid.
The photonic band gap material of the present invention may be
constructed as follows. In the most general case, a fluid, which
may be a liquid or a gas, is infiltrated into a template solid
having substantially continuous porosity throughout its extent, the
fluid is solidified, and the template solid is removed, leaving a
new solid having its solid members substantially in the positions
of the continuous porosity of the template solid and having its
pores in the positions of the solid members of the template
solid.
In one preferred embodiment, shown in FIG. 2, a template solid 20,
composed of a high melting metal which is substantially nonreactive
with the photonic band gap material in its liquid form, which may
be tungsten, is fabricated into the geometrical shape of
substantially the size and shape of the desired photonic band gap
material. In FIG. 2 the template solid 20 is shown with its solid
portion shown in white, and the voids in the template solid 20,
which extend substantially and continuously throughout its volume,
are shown in black. The template solid 20 is itself a reticulated
solid, having continuous pores and solid members composed-of the
same kind of metal. The template solid 20 may be produced either
from a specialty material with continuous porosity having
controlled pore size in the range of approximately 1 to
approximately 200 microns, or from an assemblage of individual
pieces such as wires in the form of bundles or in the form of woven
mesh which are held together by being bound with wire or by welding
at selected points, or from a combination of such parts, or from
any other substantially permeable structure. The template solid 20
is then immersed in a liquid body 30 composed of the molten form of
the photonic band gap material, which may be molten aluminum oxide,
which liquid is retained in a crucible 40 which may be made of
solid molybdenum, and which has a lid 42 which may be mad of solid
molybdenum and which is large enough to extend to the outer edges
of the crucible 40 and which is supported thereby, and which has at
least one opening 44 through which the template solid 20 may be
inserted or removed from the crucible 20 and through which the
template solid 20 may protrude and which is sufficiently large so
that there is substantially no contact between the template solid
20 and the lid 42. The thermal power required for melting and
maintaining the photonic band gap material as liquid 30 and for
heating the template body 20 and the molybdenum crucible 40 is
provided by an electric heater means, not shown, which may be
resistance heating or radio frequency induction heating, and is not
critical. A vessel, not shown, to maintain the liquid 30, template
solid 20, and crucible 40 in an inert gas environment such as
purified argon is present. A source of purified argon, not shown,
is present. After the crucible 40 is charged with solid of a
composition suitable to produce the desired photonic band gap
material from the liquid melt 30, the template solid 20 is inserted
into the crucible 40. Argon purge gas is admitted to the chamber
for a time sufficient to displace the air initially in the chamber.
The heater means is energized sufficiently to melt the charge but
not so powerfully as to melt the crucible 40 or the template solid
20. The molten liquid 30 impregnates the template solid 20 by
capillary action. If desired, an optional single crystal seed 50,
composed of substantially the same solid composition as the desired
photonic band gap material, may be brought into contact with the
impregnated template solid 20. The electrical power is then slowly
reduced, at a rate of 50.degree. C. per hour, so as to permit the
liquid 30 in the template solid 20 to solidify directionally from
the seed into the template upon cooling below its melting
temperature. The template solid 20 may also be raised slowly, at a
rate of approximately 2.5 centimeters per hour, above the level of
the liquid 30 by an elevator means, not shown, as the power is
reduced or maintained, permitting energy to be lost preferentially
from its top surface to the unheated chamber walls, not shown, and
solidifying the photonic band gap material in the template solid 20
even as the liquid 30 remains molten. At the end of such cooling
operation, and after contact between the template solid 20 and the
excess liquid 30 has been lost, power may then be removed from the
system. Other methods of infiltrating the template solid 20 such as
melting a solid body, fiber, ingot, rod or the like of material
that has the desired photonic band gap material composition so that
the liquid produced flows into the template solid 20, or pouring
the so produced liquid into the template solid 20, can also be
used. The precise method of infiltration is not critical.
After the system is cooled to room temperature, the template solid
20 containing the photonic band gap material 10 is removed from the
crucible 40. Excess liquid 30, if any, which has solidified outside
the template solid 20 may be mechanically removed, by grinding if
necessary. The template solid 20 is then chemically dissolved out
from the photonic band gap material 10 by submersion in a solution
of suitable chemical reagents maintained at 20.degree. C., not
shown, which may be held in a nonreactive vessel, not shown. In the
case of producing a photonic band gap material 10 composed of
reticulated aluminum oxide or sapphire, with a template solid 20
composed of tungsten, the solution may be aqueous hydrofluoric acid
mixed with aqueous nitric acid and maintained at approximately
20.degree. C.
In embodiments where the infiltrated fluid is fluid at room
temperature, such as the case of a fluid consisting of an epoxy,
the necessity to heat the fluid 30, the crucible 40, the lid 42,
and the template solid 20 above room temperature is obviated, and
no heating is required to be applied to form a fluid to carry out
the process. Heating to cure the epoxy may or may not be applied at
the discretion of the practitioner of the process
ALTERNATIVE EMBODIMENTS
The photonic band gap material of the present invention may be
constructed alternatively as follows. For a material such as
silicon which is a very aggressive solvent in its molten state,
there may be few or no suitable template solid materials which can
withstand the attack of the photonic band gap material in its
molten or liquid state. Accordingly, a means of infiltration which
avoids the presence of the photonic band gap material in its liquid
state can prove advantageous For a material such as silicon, or for
a compound semiconductor such as gallium arsenide, infiltration by
deposition of the solid from a gaseous medium, as is practiced in
the art of chemical vapor deposition, may be employed. Referring to
FIG. 3a, a single crystal substrate 60, composed substantially of
the same composition as the photonic band gap material which may be
silicon, is placed upon a support 70, which may be a silicon
carbide or graphite member. A heating means 80, which may be a
plurality of tungsten halogen lamps or a radio frequency coil
powered by a power supply 90, not shown, is disposed adjacent to
the substrate 60 and the support 70, and can cause both to become
heated to a controlled temperature, which is significantly below
the melting point of the substrate and the photonic band gap
material, which for silicon is 1410.degree. C. A template solid 100
is placed upon the substrate. The template solid 100, which may be
composed of tungsten or of sapphire, is a reticulated solid having
substantially continuous pores of a predetermined average size, and
may be fabricated in a manner similar to that recited above.
Considering that one of the requirements for a photonic band gap
material to interact appreciably with electromagnetic radiation is
that the photonic band gap material have a thickness of
approximately ten times the wavelength of the radiation of
interest, for a photonic band gap material operating in the visible
or the near infrared this dimensional requirement implies a
thickness of at most several tens of microns and at least several
microns. Deposited layers having these thicknesses are readily
attained by chemical vapor deposition methods in practical lengths
of time. Accordingly, the template solid 100 is prepared with a
thickness of at least ten times the wavelength of the radiation for
which the photonic band gap material is intended to operate. The
infiltration with the photonic band gap material is then carried
out by causing chemical vapor deposition to occur on the substrate
60 so as to infiltrate and envelop the template solid 100, by
reacting gaseous species, provided from a source 110 not shown,
containing the requisite chemical elements, which may be silicon,
so as to deposit solid silicon 102 which may grow as a single
crystal by epitaxy on the substrate 60. The infiltration so carried
out permits the deposition of solid photonic band gap material in
the continuous pores of the template solid 100 without formation of
liquid photonic band gap material, but rather by the direct
conversion of a gaseous species to a solid species. The deleterious
interaction of liquid photonic band gap material with the template
solid 100 is thus avoided. Upon completion of the deposition, the
template solid 100, which may be tungsten, can be removed
preferentially by oxidation in dry oxygen at a temperature of
approximately 800.degree. C., or by dissolution in a suitable
liquid medium.
Referring to FIG. 3b, which depicts a case where the template solid
100 is sapphire, deposition of silicon 120 in a quantity which is
not sufficient to completely fill the pores may be carried out, and
the template solid 100 may remain as a support for the silicon
layer or overcoating so produced. The deposited silicon 120 may be
introduced in such small quantity that the deposited silicon does
not form a continuous coating on the sapphire template solid 100
but rather deposits as a number of discrete crystallites supported
by the template solid 100, which is then not removed In the case
where the photonic band gap material is silicon, the rate of
oxidation of silicon in dry oxygen at 800.degree. C. is less than
400 Angstroms per hour, while the rate of oxidation of tungsten,
and the vapor pressure of its oxide, are high. Clearly, other
materials may be suitable as template solids 100 for still other
solids which may be deposited thereon by chemical vapor deposition
technology.
The deposition of material within the pores of the reticulated
template solid 100 may also be used as a means of controlling the
pore size within the reticulated solid. The pores in the template
solid 100 can be measured, for example by examination in an
electron microscope, and a desired thickness of deposited solid can
then be added to the template solid 100 by control of the duration
and conditions, such as temperature and gas pressures, in the
chemical vapor deposition process. By such a controlled addition of
a known amount of material, one can produce a pore size of known
dimension.
Referring to FIG. 4, yet another modification which may be
practiced is the fabrication of a composite structure by the
building up of segments composed of discrete materials.
As one example, one can begin with three template solid objects,
which for illustration may be rectangular plates. Two of the
template solids may be infiltrated with the same material 130, for
example infiltrating liquid sapphire into tungsten template solids,
and solidifying the sapphire in reticulated single crystal form.
These two infiltrated solids may then be placed in intimate contact
on either side of the third template solid, and held together by
any convenient mechanical means such as binding with tungsten wire,
or by spot welding, thereby forming a three-layered structure, and
an infiltration with chromium-doped liquid sapphire 140 may be
carried out. Since the two outside template solids are already
filled, there will be no tendency for capillary action to cause any
motion of liquid within those template solids. Capillary action
will cause the center template solid, which was not
previously-infiltrated, to be infiltrated with the chromium-doped
liquid sapphire 140, which upon solidification and removal of the
template solids as previously recited results in a monolithic
reticulated single crystal structure having sapphire layers 130 on
its outermost surfaces and reticulated chromium-doped sapphire
(ruby) 140 in a central layer between the two sapphire layers.
Alternatively, one could first infiltrate the central template
solid with chromium-doped sapphire (ruby) 140, and then add the two
outside template solids and repeat the infiltration using undoped
sapphire 130 to produce the equivalent structure. It is not obvious
to us how such a structure could be produced by a subtractive
technique such as that of Yablonovitch, as the monolithic
rectangular base material from which such a structure might be
machined, consisting of a chromium-doped sapphire portion
sandwiched between two undoped sapphire layers, is not readily
fabricated in single crystal form. Clearly, the example of a flat
geometrical shape having three regions is merely given for the
purpose of illustrating how the technique may be carried out. It is
equally possible to conduct such a process on other geometrical
shapes which can be fabricated by joining a plurality of template
solids, which may be shaped in any fashion desired, and which may
be assembled by infiltrating selected segments, adding additional
template solid segments, for example by binding, and repeating the
infiltration for the added segments with liquids produce different
solids in the added segments as compared to that infiltrated into
the predecessor segments. As an example of a second geometry, we
consider a series of cylindrical template solids, one of which is a
right circular cylinder which forms the core, and the others a
series of right circular cylindrical tubes of increasing internal
diameter, each machined to fit in intimate contact with the
external diameter of the cylinder that it may surround by being
slid onto as a sheath. Clearly the process of adding segments and
performing additional infiltrations can be repeated a plurality of
times, and the removal of the template solid may be carried out by
dissolution as long as its structure remains continuous to some
surface of the object. If necessary, one or more surface machining
steps may be carried out at intermediate points in the assembly
process to remove excess infiltrated solid which extends beyond the
physical dimensions of the template solid so as to make the
segments fit together properly as originally designed, and so as to
make the template solid accessible on some surface. It is be no
means clear how Yablonovitch would generate the monolithic solid
required to fabricate cylindrical objects having a plurality of
compositions. Clearly, yet other geometries, such as a three
dimensional solid composed of for example regular polyhedra, each
placed in intimate contact with its fellows, and fabricated by a
plurality of alternating infiltrations and additions of further
template solid segments, with mechanical removal of excess
infiltrated solid as required, can be envisioned.
As a second example, it is known that one can deposit by chemical
vapor deposition a series of compositions which can differ in
doping or even in chemical composition, as for example alternate
layers of GaAs and Al.sub.1-X Ga.sub.X As. By practicing such
chemical vapor deposition on a reticulated template solid 100 such
as in FIG. 3, one can build up a structure having regions of
differing doping, or regions having different composition, or
regions which differ in both doping and composition, which may then
be capable of operation as electronic generators or detectors of
electromagnetic radiation, such as light emitting diodes or photo
diodes, if appropriate electrical contacts are made to them.
The possibility of performing a plurality of repetitions of
introductions of template solids followed by additional chemical
vapor deposition steps so as to build up a structure of some
desired geometry is also clearly recognized, in analogy to the
discussion above regarding fabrication of structures by the use of
repeated liquid infiltration and addition of further segments of
template solid.
An example is presented which demonstrates the efficacy of the
present photonic band gap material as compared to the previous
invention of Yablonovitch et. al. Yablonovitch teaches that the
photonic band gap material produced by removing material from a
single slab of dielectric material may operate in the microwave
region of the electromagnetic spectrum. He further teaches that the
same construction method, appropriately reduced in size, as for
example by using reactive ion etching, may possibly permit
operation in the visible, but to date he has not reported any such
behavior. He states, however, in Physical Review Letters volume 63,
page 1950, dated 30 Oct. 1989, that "In the absence of any further
theoretical guidance, we adopted an empirical, Edisonian approach.
Literally, we used the cut-and-try method. Dozens of fcc structures
were painstakingly machined out of low-loss dielectric
materials."
We further recognize that the photonic band gap material that we
have produced can be tuned to operate with electromagnetic
radiation in distinct portions of the electromagnetic spectrum by
at least three methods, which modify or control the physical
dimensions of the photonic band gap material. One tuning method is
based on the physical dimensions of the pores and solid elements
that make up the reticulated photonic band gap material as it is
inherently produced. Control of such dimensions is clearly a
feature of our fabrication method, either by liquid or gaseous
infiltration in to a template solid having predetermined pore
sizes, or by subsequent control of the thickness of deposition of
solid by gaseous infiltration in a chemical vapor deposition
process. A second method and means of controlling such mechanical
dimensions of the photonic band gap material is the application (or
removal) of thermal energy to modify the dimensions of the pores
and solid elements of the material by virtue of the thermal
expansion coefficient of the material, in which maintaining the
material at a deliberately chosen temperature will cause a
consequential predictable increase or diminution of the dimensions
of the material through the action of the thermal expansion
coefficient. A third method and means of controlling the mechanical
dimensions of the photonic band gap material is the deliberate
application of mechanical force or pressure to selected free
surfaces of the photonic band gap material, to modify the
dimensions of the pores and solid elements of the photonic band gap
material by the effects of the compressibility of the material from
which the photonic band gap material is constructed, in which
application of a mechanical force or pressure to selected free
surfaces of the photonic band gap material leads to reproducible
mechanical distortions of the photonic band gap material, in a
manner similar to the distortions that a spring exhibits when
mechanical forces of compression or tension are applied to its
ends, thereby changing its mechanical dimensions. Clearly, any
other method of introducing a deliberate change in the dimensions
of the photonic band gap material, such as electrostriction, or
magnetostriction, if the material can support such a method, would
work equally well as a means of tuning the material to respond in a
selected region of the electromagnetic spectrum. We envision the
possibility of dynamically tuning the photonic band gap material,
either for continuous use at a particular range of frequencies or
wavelengths in analogy to analog electronic technology, wherein the
photonic band gap material operates in a continuous manner with
some proportion to the intensity or value of the electromagnetic
radiation, or to permit the modulation of such a use in analogy to
digital electronic technology, wherein the photonic band gap
material can be made to switch its behavior on and off with regard
to a particular range of frequencies or wavelengths.
Beginning with the first photonic band gap material built according
to our discoveries, we found that we could observe optical behavior
in the visible. We can further point out that using our fabrication
methods, we can for example produce a monolithic photonic band gap
material having larger pores in its interior and smaller ones in
its exterior by the simple expedient of stacking template solids
together having the requisite dimensions in any order we choose.
Yablonovitch does not have a mechanism to fabricate such a device
or structure by his cutting method alone.
Obviously many modifications and variations of the present
invention are possible in light of the above teachings. The
photonic band gap material, and its fabrication procedures
described herein provide a photonic band gap material which
functions in the near infrared and visible portions of the
electromagnetic spectrum. Other techniques for applying the present
invention may be employed and changes made in regard to some of the
details and still provide a photonic band gap material without
deviating from the spirit and scope of the invention specified
herein.
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